BG migration for populating stage event hash

What does this MR do?

DO NOT MERGE, THIS CHANGE MUST GO INTO 14.3

This change adds BG migration to backfill the stage_event_hash_id column for the Value Stream Analytics group and project stages and adds a not-null constraint (the application already handles this since 14.2).

I tested the backfill logic via PG.ai and the updated records are correct:

Analytics::CycleAnalytics::GroupStage.find_each.map { |g| g.stage_event_hash.hash_sha256 == g.events_hash_code }.uniq

DB

The migration upserts a few hundred rows and then updates about 2000 group stage records one by one. Due to the low record count, I'm not using batch updates here. Self-managed will have significantly fewer rows. As of today, there are not project stage records however, I run the migration for the project_stages table, just in case.

Up:

== 20210731132939 BackfillStageEventHash: migrating ===========================
-- current_schema()
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE analytics_cycle_analytics_group_stages\nADD CONSTRAINT check_e6bd4271b5\nCHECK ( stage_event_hash_id IS NOT NULL )\nNOT VALID;\n")
   -> 0.0007s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE analytics_cycle_analytics_group_stages VALIDATE CONSTRAINT check_e6bd4271b5;")
   -> 0.0012s
-- current_schema()
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE analytics_cycle_analytics_project_stages\nADD CONSTRAINT check_8f6019de1e\nCHECK ( stage_event_hash_id IS NOT NULL )\nNOT VALID;\n")
   -> 0.0007s
-- current_schema()
   -> 0.0002s
-- execute("ALTER TABLE analytics_cycle_analytics_project_stages VALIDATE CONSTRAINT check_8f6019de1e;")
   -> 0.0010s
== 20210731132939 BackfillStageEventHash: migrated (0.0382s) ==================

== 20210731132939 BackfillStageEventHash: reverting ===========================
-- execute("ALTER TABLE analytics_cycle_analytics_group_stages\nDROP CONSTRAINT IF EXISTS check_e6bd4271b5\n")
   -> 0.0009s
-- execute("ALTER TABLE analytics_cycle_analytics_project_stages\nDROP CONSTRAINT IF EXISTS check_8f6019de1e\n")
   -> 0.0007s
== 20210731132939 BackfillStageEventHash: reverted

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

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Related to #335291 (closed)